The present invention relates to an endoscope apparatus using a solid-state image pickup device which is inserted in a body cavity to pick up an image thereof.
A typical conventional endoscope apparatus uses a fiberscope having an image guide with a bundle of optical fibers. In order to display an image obtained by such an endoscope on a TV monitor and use it for a medical diagnosis, an image at an eyepiece of the fiberscope is picked up by a TV camera and displayed on the TV monitor.
An improved conventional endoscope has been proposed along with the development of a very compact solid-state image pickup device in recent years. In such an endoscope, the solid-state image pickup device is mounted in the distal end of an insertion portion of the endoscope, and an image of an object to be examined is directly picked up and extracted as a video signal without using an image guide fiber. The extracted video signal is displayed as an image on a TV monitor. Image observation and medical diagnosis according to this apparatus are performed through the TV monitor.
A typical example of a conventional solid-state image pickup device is shown in FIG. 1.
Referring to FIG. 1, the solid-state image pickup device comprises independent light-receiving portions P11, . . . Pmn, vertical transfer portions CV1, . . . CVm, gates G11, . . . Gmn, and horizontal transfer portion H. Upon reception of light on light-receiving portions P11, . . . Pmn, signal components charged by light-receiving portions P11, . . . Pmn during a field or frame period are transferred to vertical transfer portions VC1, . . . VCm through gates G11, . . . Gmn. When signal transfer is completed, new signal charging is started. The transferred charges are sequentially sent to horizontal transfer portion H and extracted outside the image pickup device according to a TV (scanning) scheme. Since the charges are stored in light-receiving portions P11, . . . Pmn of the image pickup device according to the intensity of incident light, a portion of an object to be examined need not be illuminated by continuous light but can be by light pulses.
Illumination of an object portion according to the conventional apparatus is performed, as indicated by an illuminated light waveform and a video signal waveform in FIGS. 2A and 2C. Light pulses are respectively generated at start timings of the fields. An optical charge stored by the light-receiving portions during each field period is transferred to the vertical transfer portions during the transfer period within the blanking period and is then extracted as a video output through the horizontal transfer portion during the next field period.
In the case of extracting signals from the solid-state image pickup device according to interlaced scanning, signals are simultaneously extracted from the vertically adjacent light-receiving portions and are added to each other to increase signal-to-noise ratio (S/N). More specifically, the signals each generated by adding the charges from two light-receiving portions vertically aligned are simultaneously transferred to a vertical transfer portion. In this case, even if the number of light-receiving portions corresponds to a total number of pixels of one frame, signals are extracted from the light-receiving portions in units of fields and thus the storage period is one field period (normally 1/60 sec). The solid-state image pickup device having the light-receiving portions whose storage period is one field period is known as a field storage mode solid-state image pickup device.
Diagnosis and therapy using an endoscope are performed such that a doctor observes an image on a real-time basis. An image may be photographed to perform an objective diagnosis and to check an effect of the treatment.
In an endoscope using a fiberscope, a camera is attached to the eyepiece of the probe to take a picture of the object to be examined. In this case, underexposure is compensated by a flash light source or the like, and thus an exposure time of 1/125 sec or less must be substantially used.
In an endoscope using a solid-state image pickup device, however, since an image on the TV monitor is photographed as a picture, the display image is normally frozen (i.e., the display image is temporarily processed to obtain a still image). More specifically, a frame memory is arranged in a video processor for processing an image signal, and one-frame image data is stored in the frame memory. The stored image is repeatedly read out to obtain a still image. In order to store a one-frame image in the frame memory, the write time is 1/30 sec corresponding to a two-field period according to the normal interlaced TV scanning.
In a conventional endoscope apparatus using a solid-state image pickup device, since a light pulse is emitted at the start timing of each field period, as previously described, an image of two fields constituting one frame is lagged by time interval (1/60 sec since the one-field period is 1/60 sec) T1 corresponding to the one-field period, i.e., an interval between two pulses for obtaining an image. For this reason, a one-frame still image formed by the two fields during freezing is blurred because of a lapse of time interval T1.
According to still another conventional endoscope apparatus, beams of the three primary colors, i.e., red (R), green (G), and blue (B) are sequentially emitted onto the photographing portion in units of fields. Image data derived by these color components from the solid-state image pickup device is stored in the respective color memories. Output signals from these memories are mixed to obtain a one-frame color image signal. This endoscope apparatus is called a field sequential color image pickup type endoscope apparatus. In this apparatus, if continuous light instead of light pulses is used, the period of three fields (1/20 sec if the one-field period is 1/60 sec) is required to store a signal of three color components, i.e., one frame, in a memory. For this reason, if the image is frozen, a time lag occurs by the differences between the timings of images of the respective color components. It is difficult to obtain a good frozen image for a moving image. As a result, a still image of high quality cannot be recorded. It is also possible to use light pulses in place of continuous light to illuminate the portion to be photographed. As shown in FIGS. 3A, 3C, and 3D, if light pulses are emitted in synchronism with the field period at the start timings of the fields, period T1' (1/30 second if the one-field period is 1/60 sec) substantially corresponding to the two-field period is required to obtain a one-frame image. As a result, a still image of high quality cannot be obtained.